The subject matter of this invention relates to combustion systems and methods. In particular, the present invention is directed to systems and methods for combusting moisture-ladened fuels, such as biomass, in steam generating boilers.
Pulverized coal is a conventional fuel for furnaces or steam generating boilers. It is desirable to replace or supplement pulverized coal with substitute fuels in certain circumstances. Pulverized coal typically has a mass mean particle size of the order of 30-40 microns that allows it to substantially completely burn in suspension in the 1-2 seconds of residence time in the combustion zone. However, many substitution fuels, such as various biomass fuels, cannot be ground as finely as coal; with the result that mean particles sizes are of the order of 1 mm (1000 microns), with many particles as large as several mm. Combustion of these larger substitute fuels is typically both unstable and incomplete. Moreover, most of these fuels have higher moisture content and lower specific energy content than coal and, hence, are not capable of generating a high enough flame temperature to maintain the design heat transfer of the boiler system. Costly de-rates in boiler steam output are thus incurred.
Other solid fuels, such as petroleum coke, for example, have very low volatile content and, hence, these fuels are very difficult to ignite within the short period normally allotted for this purpose in conventional in-flight combustion systems. Finally, in some cases, it is desirable to forego the step of pulverization of coal and thus reduce both capital equipment and power costs associated with this preparatory step. In such cases, the instant invention provides a means of achieving complete combustion in conventional boilers and furnaces with the larger particle size. Modeling has shown that flame stability and complete in-flight combustion of such relatively large and high moisture substitute fuels can be achieved through the pre-liberation of moisture and preheating of fuel particles prior to injection into a furnace or boiler. When such particle pre-heating and drying is enhanced with oxygen, the result is not only stable and complete combustion, but also raising of flame temperatures and restoration of design boiler heat transfer rates. Attempts to date of utilizing these non-conventional fuels as the sole energy source in furnaces or boilers designed for conventional in-flight combustion have largely resulted in incomplete; i.e., inefficient and/or dangerously unstable combustion. As such, most commercial applications of in-flight combustion where non-conventional fuels are utilized have been accomplished with only fractional amounts of the total energy being supplied by the alternative fuel.
Tangential or cyclonic burners have been employed in the past to attempt to solve the problem of drying, devolatilization and combustion of solid fuels. Morgan et al. (Morgan, D., Biffin, M., No, S. Y., and N. Syred, “An Analysis of the Behavior of Non-Slagging, Coal Fired, Cyclone Combustors Using a Phenomenological Model”, Twenty Second Symposium (International) on Combustion/The Combustion Institute, 1988, pp. 175-182) describes an air-fuel, non-slagging (i.e., one in which the solid fuel residue does not melt within the reactor) cyclone reactor for combustion of solid fuel in which the solid fuel residue is discharged from a separate conduit than the gaseous products of solid fuel combustion. However, testing showed large percentages of unburned fuel leaving with the solid residue stream under the relatively low-temperature conditions needed to prevent fuel slagging. See FIG. 3 of Morgan et al. which indicates a steep decrease in carbon burnout for particle diameter greater than 500 microns (½ mm). This is an inherent limitation of the solid/gaseous separation that occurs in the Morgan et al. device. Sarv et al. in International Application No. WO2008/151271, D'Agostini et al. in U.S. Pat. No. 6,910,432, U.S. Pat. No. 6,968,791 and D'Agostini in U.S. Patent Application Publication No. 2012/023823, each of which are hereby incorporated by reference in their entirety, disclose an oxygen-enriched slagging cyclone combustor in which a molten solid residue (slag) and gaseous products of combustion are discharged in separate streams from the combustor. The U.S. Pat. No. 6,968,791 patent and the US 2012/023823 application also comprise injection of secondary fuel with oxygen along the main axis of the cyclonic reactor. Slayter in U.S. Pat. No. 2,455,907, which is hereby incorporated by reference in its entirety, discloses a melting apparatus for forming glass fibers in which a solid glass batch material is fed vertically downward along the axis of the melter. An air-fuel gas mixture is injected at the top of the apparatus to create a swirling flame that heats and melts the glass batch material. Both the gaseous products of air-gas combustion and the molten glass fall downward into a converging conical section from which they are discharged into a forehearth for glass refining to be accomplished.
None of the prior art references discuss the criticality of the dimensional proportion of the features within the reaction chamber nor, in particular, the unique and necessary flow field resulting from adherence to the requisite proportions. In fact, the dimensional proportions of the features within the reaction chamber are of very low criticality for the Morgan et al. device, the devices disclosed in WO2008/151271, the device disclosed in U.S. Pat. No. 6,910,432, the device disclosed in U.S. Pat. No. 6,968,791, and US 2012/023823. This is because the separation of solid and gaseous phases taking place within the respective devices de-couples the fluid motion of the gaseous and solid phases. The dimensional proportion of the features within the melting apparatus are not highly critical either within the U.S. Pat. No. 2,455,907 because the glass fibers melt along the walls of the melting apparatus and the molten material flows naturally under the action of gravity toward a bottom discharge port. Moreover, there is no reference to the importance of the relative dimensions among the discharge opening, the apparatus (melting chamber) diameter and the flow properties within the device.
A system and method for liberating moisture from and combusting solid fuel having relatively high moisture contents that is efficient and has high-flame stability and complete in-flight combustion would be desirable in the art.